Computer Peripherals School of Computer Engineering Nanyang Technological University Singapore These notes are part of a 3rd year undergraduate course called "Computer Peripherals", taught at Nanyang Technological University School of Computer Engineering in Singapore, and developed by Associate Professor Kwoh Chee Keong. The course covered various topics relevant to modern computers (at that time), such as displays, buses, printers, keyboards, storage devices etc... The course is no longer running, but these notes have been provided courtesy of him although the material has been compiled from various sources and various people. I do not claim any copyright or ownership of this work; third parties downloading the material agree to not assert any copyright on the material. If you use this for any commercial purpose, I hope you would remember where you found it. Further reading is suggested at the end of each chapter, however you are recommended to consider a much more modern alternative reference text as follows: Computer Architecture: an embedded approach Ian McLoughlin McGraw-Hill 2011 Chapter 16 Thermal Printing 16.1 Introduction The growth in use of personal computers in the work place and in the home has spurred the development of compact printers capable of printing text and graphics for a variety of applications. A number of competing technologies have emerged, each with its own characteristics, and among these, thermal printing has progressed from providing primarily a utility function confined to special paper for calculators or portable terminals, to a much wider range of applications and capabilities. Today's market offerings range from low cost, quiet, home printers using a transfer ribbon to provide solid black or colour printing on selected smooth bond-like paper, to machines capable of printing several pages a minute at densities of 200 per inch and higher. With the advent of the resistive ribbon technology (see below), the capability extends to high quality correspondence printing on a wide range of papers. The simplicity and reliability of the technology has made thermal printing also of special interest for instrumentation, ticket and label making devices. The next major development of thermal printing was in the 1970s, when silicon mesa printheads and the thick film were designed as serial printheads to output a complete character at a time. The silicon mesa was the technology employed in the Texas Instruments Silent 700 terminal and the Hewlett Packard computer series, (Lo and Keil, 1980). There was also a flurry of activity in Japan to develop thermal printing for facsimile applications. The demand for printing on plain paper led to the development of thermal transfer printing. The next logical evolution was the capability to print on a wide range of papers, and this led to the introduction in 1984 of resistive ribbon printing technology. The 1980s have seen a full blossoming of the thermal transfer technology, and today there are many dozens of printers in the market place, ranging from basic serial devices to high resolution machines for full colour printing and copying. 16.2 Technology Attributes Thermal technology provides a simple mechanism for making a compact printer. It is characterised as a quiet, highly reliable, rugged, clean, mechanically simple, relatively low power (as compared to impact) and attractively low cost method of printing that is adaptable to serial or line printhead configurations. The technology is extendible, and it is now also being developed for high quality images in full colour. There are essentially three different printing methods employing thermal technology. Direct thermal printing on heat sensitive paper was an early method employed that continues to find application due to its simplicity. The introduction of thermal transfer printing, in www.lintech.org Thermal Printers 2 which a sheet or ribbon of a very thin paper or plastic material carrying a layer of waxy, easily melted ink is interposed between the printhead and the paper, made possible dense black printing on non-chemically coated, but distinctly smooth paper. In a significantly different development known as resistive ribbon, heat is liberated not in the printhead but in a conducting ribbon by injection of current from the printhead. This is the technology employed in the IBM Quietwriter serial printers that provide high quality text and graphics on a wide range of papers. The basic components of thermal printing are the printhead and the receiving medium, which in the case of direct thermal is a heat sensitive paper, and in thermal transfer is both an intermediate ribbon and the paper. The different printer types can employ either serial or line printheads, with black or Gary scale capability, and today may also provide print in full colour. In the serial configuration, the printhead contains a vertical column of heater elements covering the height of a character box and printing is carried out by moving the head across the page to output a line of text at a time, as shown in Figure 1 Configuration for serial printing on heat sensitive paper. In another configuration, a line head containing several thousand closely spaced heater elements spans completely across the page as shown in Figure 2 Arrangement for thermal transfer printing using a line printhead, transfer ribbon and plain paper. Now, the paper moves at a fixed rate beneath the stationary printhead, and the printer contains remarkably few moving parts. In either case a thermally sensitive paper or a transfer ribbon may be used. Multicolour is achieved by using a ribbon with multiple colour serial stripes, and producing a Gary scale involves spatial dithering of the printing elements. The recent full colour printers use spatial dithering and also provide optical density modulation by using sublimable dyes as the transfer medium. Figure 1 Configuration for serial printing on heat sensitive paper. Figure 2 Arrangement for thermal transfer printing using a line printhead, transfer ribbon and plain paper. www.lintech.org Thermal Printers 3 A factor that has helped considerably in the steady development of the technology has been that the printhead could be fabricated by thin layer deposition and lithographic techniques familiar to the electronics industry. With logic and drive circuits integrated upon the printhead, a reliable, cost effective printhead technology has emerged to make viable a range of printers which are competitive in the marketplace. Some basic functional limitations, for example, print speed, or the inability to print on a wide range of surfaces, exist that may be traced to fundamental properties of the thermal process. 16.3 Thermal Printing Technology Thermal printing is characterised by the utilisation of heat to produce a chemical or physical change so as to give rise to visible marks upon paper or some other substrate material. Although it is appealing to consider generation of the heat pattern optically, for example, by a scanning laser beam, this is an inefficient way to deliver the energy required for printing at useful rates. Printheads for thermal printing use instead the conversion of electrical energy into thermal energy by means of Joule heating within an electrically resistive material. Figure 3 Expanded view of heating element structure, and cross-section of thick film technology serial printhead. shows schematically some structural details of a serial thermal printhead containing an array of electrical resistor heating elements constructed upon a ceramic substrate. The resistors are obtained by making contact to a line of resistive material prepared by so called thick film techniques, in which the resistive line is deposited in the form of a paste which is then fired in a furnace to develop the required characteristics. The resistor elements may be made by either thick film or thin film deposition methods. Thick film elements provide a solid footprint for making contact to the paper; however, thin film technology makes possible the use of lithographic patterning to make high resolution resistor arrays, in which the resistors may be given, if desired, an optimum shape. This is the case for the page wide head where a resolution of 200 printing elements per inch is the norm and where much higher resolution is becoming increasingly prevalent. Figure 3 Expanded view of heating element structure, and cross-section of thick film technology serial printhead. A further distinction is made between heads which have the electrodes placed extremely close to the edge of the printhead for ease of peel of the transfer ribbon. In order to improve www.lintech.org Thermal Printers 4 the mechanical contact between the head and the paper or ribbon a narrow strip of a glaze material (partial glaze) is placed beneath the thin film resistor heating element. Figure 4 Photolithographically defined heating elements of a thin film technology printhead. shows an example of a thin film heater arrangement where the resistor elements have been given a meandering shape by photolithographic etching. An important aspect is that the printhead is given a protective coating to minimise wear since in use it is brought into close mechanical contact to either a special paper or a thermal transfer ribbon. Figure 4 Photolithographically defined heating elements of a thin film technology printhead. 16.3.1 Direct Printing on Thermal Paper. We consider first the direct thermal printing case as shown schematically in Figure 5 Direct printing on heat sensitive paper., where a printhead is used to supply heat by contact to a heat sensitive paper. A pulse of current is applied to one or more of the heating elements to cause rapid heating of the resistors; heat conducted to the paper via the region of contact produces a significant temperature rise on the surface of the heat sensitive paper. The resistor element must be able to heat the thermally sensitive paper up to the coloration temperature, and its surface must be highly resistant to friction from the paper.